1. Field of the Invention
The present invention relates to a semiconductor laser and a multi-semiconductor laser which have a narrow far field pattern (hereafter, referred to as a FFP) by a low threshold value electric current.
2. Description of the Related Art
As a low threshold electric current semiconductor laser, there is a SDH (Separated Double Heterostructure) laser. The SDH laser is, as a schematic cross-sectional diagram thereof is shown in FIG. 1, such that a first conductivity-type, that is, a GaAs substrate 1 whose one major surface of a p-type is made a (100) crystalline surface is prepared and on the one major surface by the (100) surface is formed a straight stripe-shaped ridge 2 extending in a [110] axis direction and on the major surface of the substrate 1 having the ridge 2 are epitaxially grown, by turns, a buffer layer 3 consisting of a p-type GaAs of the first conductivity-type, a first cladding layer 4 consisting of a p-type AlGaAs, an active layer 5 by, for example, a multi-quantum well structure, a second cladding layer 6, 8 consisting of an n-type AlGaAs of a second conductivity-type, an AlGaAs electric current layer 7 of a pnp structure in which, for example, a p layer, an n layer and a p layer are superimposed by turns and a capping layer 9 consisting of an n type GaAs of the second conductivity-type, by an MOCVD (Metal Organic Chemical Vapor Deposition) method.
In the above-mentioned MOCVD of respective semiconductor layers, methyl system organic metals are used as raw materials. In the case, there occurs a non-growth surface of {111} B surfaces on the ridge 2 and on the {111} B surfaces occurs a fault between a semiconductor layers which have grown on grooves on both side of the ridge 2 and the B surface and a cross-sectional triangle shaped semiconductor portion 10 sandwiched between inclined surfaces by the {111} B surfaces is formed on the ridge 2.
A semiconductor laser by the SDH structure is capable of forming a narrow width active layer 5 in a cross-sectionally triangle shaped semiconductor portion 10 on the ridge 2, and also, since the AlGaAs electric current blocking layer 7 of the pnp structure can be formed on the both side thereof, it is possible to effectively supply an electric current into the active layer 5 of the semiconductor portion 10 to thereby carry out laser oscillation.
Further, in the arrangement, since the electric current blocking layer 7 has the same band gap as that of the cladding layer, a light which has generated in the active layer 5 of the semiconductor portion 10 is trapped in a lateral direction (a width direction of the stripe), thereby making it possible to carry out reduction of a threshold electric current 1th.
By the way, as in the SDH type laser, a semiconductor laser, which has the straight stripe active layer, and in which the width of the active layer is particularly narrow and lowering of the low threshold value is realized has a large FFP pattern and a tendency of a spreading angle of the laser beam becoming large.
Then, in the semiconductor laser with the laser beam having the large spreading angle, a lens with a large numerical aperture NA becomes necessary, and when a lens with a small numerical aperture is used, efficiency in the use of light is lowered.
On the other hand, a semiconductor laser for reducing the FFP pattern is proposed in a Japanese laid-open patent publication No. 6-334255. The semiconductor laser i s, as its schematic cross-sectional diagram is shown in FIG. 2, such that in the above-mentioned SDH type laser structure of FIG. 1, a width in the vicinity of an end surface in a longitudinal direction of the resonator of the stripe-shaped ridge 2 is made wide, and in response to this, a width at an end portion of the active layer 5 of the cross-sectionally triangle shaped semiconductor portion 10 is made wider than that in the central portion, thereby making small the FFP pattern of a laser light generate d from the end portion. Meanwhile, in FIG. 2, an overlapping explanation will be omitted by attaching the same referential numerals to the portions corresponding to those in FIG. 1.
However, since the threshold value electric current of a semiconductor laser depends on the width of an active layer forming a resonator, in a case where the width in the vicinity of the end surface of the resonator is, as mentioned above, made wider, a value of an electric current value for oscillation at the active layer in the vicinity of the end surface becomes larger than the central portion, and as a result, the threshold value for oscillation electric current becomes higher compared with an SDH type the laser which has a uniformly narrow stripe width.
Particularly, in a case where a multi-semiconductor laser in which a plurality of laser elements are arranged and integrated on a same substrate is arranged, when the threshold electric current becomes higher in the laser element, heat generation becomes large, and due to a heat influence on the neighboring semiconductor laser elements, that is, mutual heat interference, there occurs a change in a laser oscillating characteristic in each element, a reduction in reliability as well as a life cycle, thereby exerting an influence on the characteristic of the multi-semiconductor laser.
Then, in the multi-semiconductor laser, it is desirable that the threshold electric current becomes as smaller as possible in each semiconductor laser element.
Also, as a light source of, for example, a laser light printer and the like, there is a case in which there is a demand that a droop characteristic be limited to within several percent. In this case, there is used such a technique in which a reflection factor Rf at a front end surface forming a front light emitting end which emits an inherent laser light is made higher than a reflection factor Rr at a rear end surface on an opposite side therefrom, that is, Rf greater than Rr is satisfied.
Then, when the reflection factor Rr at the rear end surface is made small and further, the width of the resonator at the rear end surface is made wider, the emitting light quantity from the rear side becomes large and further, its emitting angle becomes small.
By the way, in generally driving the semiconductor laser, for example, its power control is such that the intensity of the rear emitting light from the rear of the semiconductor laser proportionate to the front emitting light is detected by a photo-detector element, generally, a photo-diode and the detected output is used as a power control signal by monitoring the power of the front emitting light.
But, when the amount of the rear emitting light is large and the emitting angle is small, as mentioned above, there entails an efficient incidence of a strong laser light into the photo-diode, and absorption of the light becomes saturated, thereby incurring a problem that correct monitoring of light output can not be carried out.
An object of the present invention is to make the FFP pattern small and further, to implement a reduction of the threshold value of electric current. Further, by implementing a reduction in light density of the rear side emitting laser light, the saturation in the detection of the rear side emitting laser light for monitoring the output by, for example, the semiconductor laser is to be avoided.
According to an aspect of the present invention, there is provided a semiconductor laser in which a stripe portion extending in an  less than 011 greater than  crystal axis direction of a compound semiconductor substrate wherein a {100} crystal surface is made a major surface is formed between stepped portions and in the stripe portion is arranged a laser resonator and the width of the stripe portion is made wider on one end surface side compared with those in the central portion as well as on the other end surface side. Then, in accordance with the shape of the stripe portion, the stripe shape of an active layer of a laser resonator is made wider compared with those in the central portion and at the other end surface.
Also, according to another aspect of the present invention there is provided a multi-semiconductor laser which is arranged such that a plurality of semiconductor laser elements by the above-mentioned arrangement of the semiconductor laser according to the present invention are disposed on a common compound semiconductor substrate.
In the semiconductor laser according to the present invention, since the width is made wider only at the one end surface of the resonator, the FFP pattern is scaled down with respect to the front emitting laser light is concerned, and with respect to the other end surface, as a required narrow width arrangement, an increase in a threshold value electric current is alleviated and also, with respect to the rear emitting laser light, since the resonator is not arranged thereby, the emitting angle of the rear emitting laser light is made large to thereby avoid saturation in detecting a monitoring laser light.
Then, in a multi-semiconductor laser, by alleviating an increase in the threshold value electric current, mutual heat interferences among the laser elements is to be avoided.